Steam turbines typically comprise static nozzle segments that direct the flow of steam into rotating buckets that are connected to a rotor. In steam turbines, the nozzle including the airfoil or blade construction is typically called a diaphragm stage. Conventional diaphragm stages are constructed principally using one of two methods. A first method uses a band/ring construction wherein the airfoils are first welded between inner and outer bands extending about 180°. Those arcuate bands with welded airfoils are then assembled, i.e., welded between the inner and outer rings of the stator of the turbine. The second method often consists of airfoils welded directly to inner and outer rings using a fillet weld at the interface. The latter method is typically used for larger airfoils where access for creating the weld is available.
There are inherent limitations using the band/ring method of assembly. A principle limitation in the band/ring assembly method is the inherent weld distortion of the flowpath, i.e., between adjacent blades and the steam path sidewalls. The weld used for these assemblies is of considerable size and heat input. That is, the weld requires high heat input using a significant quantity of metal filler. Alternatively, the welds are very deep electron beam welds without filler metal. This material or heat input causes the flow path to distort e.g., material shrinkage causes the airfoils to bow out of their designed shaped in the flow path. In many cases, the airfoils require adjustment after welding and stress relief. The result of this steam path distortion is reduced stator efficiency. The surface profiles of the inner and outer bands can also change as a result of welding the nozzles into the stator assembly further causing an irregular flow path. The nozzles and bands thus generally bend and distort. This requires substantial finishing of the nozzle configuration to bring it into design criteria. In many cases, approximately 30% of the costs of the overall construction of the nozzle assembly is in the deformation of the nozzle assembly, after welding and stress relief, back to its design configuration.
Also, methods of assembly using single nozzle construction welded into rings do not have determined weld depth, lack assembly alignment features on both the inner and outer ring and also lack retainment features in the event of a weld failure. Further, current nozzle assemblies and designs do not have common features between nozzle sizes that enable repeatable fixturing processes. That is, the nozzle assemblies do not have a feature common to all nozzle sizes for reference by machine control tools and without that feature each nozzle assembly size requires specific setup, preprocessing, and specific tooling with consequent increase costs. Accordingly, there has been demonstrated a need for an improved steam flowpath for a stator nozzle which includes low input heat welds to minimize or eliminate steam path distortion resultant from welding processes as well as to improve production and cycle costs by adding features that assist in assembly procedures, machining fixturing, facilitate alignment of the nozzle assembly in the stator and create a mechanical lock to prevent downstream movement of the nozzle assembly in the event of a weld failure.